The present invention relates to a radio frequency communications system and method, and more particularly to a two-way single sideband, land mobile system in which a pilot tone is transmitted with an audio signal and a phase lock loop used to acquire the pilot tone and thus the audio signal.
Known prior art systems of this type have pre-emphasized the audio signal prior to compression. However, the desired degree of pre-emphasis has been difficult to achieve. In one aspect, the present invention greatly simplifies the pre-emphasis circuit by compressing the audio signal before pre-emphasis.
Automatic level control circuits (ALC) are well known and generally operate to maintain a constant peak output power from the transmitter. Since the output power of a single sideband transmitter is a function of the amplitude of the modulation signal applied thereto, the output power of known transmitters tend to widely fluctuate as a result of the widely varying characteristics of a typical audio signal. In another aspect, the present invention obviates this problem by adjusting the power of the transmitter only during an initial time interval when a constant amplitude signal is present, and thereafter maintaining the gain of the transmitter constant. A limiter in the audio circuits of the transmitter thereafter limits the amplitude of audio signals and thus prevents the transmitter power output from exceeding its rated value.
In generally known systems, the pilot tone may be masked by audio frequency components, or alternatively the phase lock loop of the receiver may try to lock on a portion of the audio signal making initial acquisition difficult. In one aspect, the present invention obviates this problem by transmitting only the pilot tone for a period of time sufficient for acquisition thereof by the phase lock loop of the receiver. Once acquisition has been achieved, the frequency response characteristics of the pilot tone filter are narrowed and the pilot tone attenuated to avoid possible interference with the audio signal without loss of lock-on.
Frequency modulation of the pilot tone for tone coded squelch purposes is known. In this way, the audio signal of a particular receiver may be gated off to avoid extraneous noise until such time as a uniquely coded pilot tone is received. In a further aspect, the present invention achieves frequency modulation of the pilot tone by locating the modulating source in the return end of the loop filter of a phase lock loop. Simplicity of circuit design may thus be achieved when the frequency of modulation is high with respect to the bandwidth of the loop filter associated with the phase lock loop.
In receivers in systems of the type heretofore described, the frequency response characteristics of the loop filter are varied as a function of lock-on of the phase lock loop. In this way, the pilot tone may be rapidly acquired and thereafter maintained in the event of the temporary fades characteristic of two-way, land mobile communications. In another aspect, the present invention improves upon this feature by detecting the strength of the detected pilot tone immediately prior to loss of lock-on, and increasing the delay in reverting to the rapid acquisition mode under conditions where the signal is weak and fades are likely to be longer in duration.
In generally known prior art receivers, the amplitude of the pilot tone is detected and used to control the gain of the receiver, i.e., to adjust the strength of the composite audio and tone signal to bring the tone signal up to a predetermined level Since the amplitude of the pilot tone is being adjusted in such receivers in response to detection of the pilot tone, undesirable "pumping" may result. This problem is avoided in the present invention by using the signal strength of the detected pilot tone to control only the gain of the audio signal components of the composite signal.
In generally known systems, the speed of response in acquisition of the pilot tone is a function of the bandwidth of the pilot filter As explained in connection with the transmitter of the present invention, the initial transmission of a full power, unattenuated pilot tone greatly facilitates lock-on. Thereafter, the pilot filter may be switched to a narrow band mode and the amplitude of the pilot tone reduced without the loss of lock-on as a result of the presence of a high amplitude audio signal. Thus, the present invention controls the frequency response characteristics of the pilot filter as a function of phase lock loop lock-on.
In addition, the amount of delay in switching to the acquisition mode following loss of lock-on may be adjusted as a function of signal strength immediately prior to the loss of lock-on. The amount of delay in switching to the wideband pilot filter is always greater than the amount of delay in switching to the wide-band phase lock loop filter. This allows the loop to make rapid corrections if the received pilot signal drifts in frequency without increasing the pilot filter bandwidth and thus subjecting the loop to possible interference from audio components of the received signal.
An additional problem in generally known receivers is the acquisition of the pilot tone in the presence of an audio signal. As earlier explained, the present invention transmits the pilot tone only during an initial time interval In addition, the frequency response characteristics of the wideband pilot tone filter are desirably selected such that noise tends to drive the oscillator associated with the phase lock loop to one extreme, thereby tending to center the pilot tone in the bandwidth of receiver's IF filter (the primary selectivity element) In this way, the presence of the tone is immediately detected even if that tone is not exactly on the expected frequency. This minimizes the requirement for oscillator stability in the transmitter and receiver and this reduces cost and complexity.
In systems where the amplitude of the pilot tone is less than the amplitude of the peak permissible audio signal, the presence of an audio signal in excess of the limit will reduce the amplitude of the pilot tone components Where the dip in pilot tone amplitude is detected by the receiver to control squelch ad the AGC circuit, undesirable dead spots or gaps in the audio signal may result. One known prior art method of solving this problem is the introduction of delay circuit to prevent operation of the squelch circuit during audio peaks However, such delay also applies at the end of transmissions and results in annoying noise bursts.
In the present invention, the dead spots are avoided without the need for a delay circuit. This is accomplished by weighting and summing the audio and pilot signals to maintain a nearly constant amplitude signal to the squelch detector in spite of varying audio and pilot signal levels.
The foregong and many other features, objects and advantages of the present invention will be readily apparent to one skilled in this art from the claims and from a perusal of the following specification when read in conjunction with the appended drawings.